Effect of Gd3+ doping and reaction temperature on structural and optical properties of CdS nanoparticles

Materials Science and Engineering B 200 (2015) 59–66

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Effect of Gd3+ doping and reaction temperature on structural and optical properties of CdS nanoparticles Gajanan Pandey a,∗ , Supriya Dixit b , A.K. Shrivastava b a b

Department of Applied Chemistry, Babasaheb Bhimrao Ambedkar University, Lucknow, 226025, (U.P.), India School of Studies in Physics, Jiwaji University, Gwalior, 474011, (M.P.), India

a r t i c l e

i n f o

Article history: Received 1 December 2014 Received in revised form 27 May 2015 Accepted 17 June 2015 Available online 29 June 2015 Keywords: Gd:CdS NPs Aqueous synthesis XRD Electron microscopy Optical properties Photoluminescence

a b s t r a c t CdS and Gd3+ ions doped CdS nanoparticles have been prepared at two reaction temperatures 90 and 120 ◦ C in aqueous medium in presence of cationic surfactant cetyltrimethylammonium bromide. X-ray diffraction study revealed predominant formation of zinc blend CdS and Gd:CdS at 90 ◦ C, while at 120 ◦ C, phase pure wurtzite CdS and Gd:CdS were formed. From EDX spectra and ICP-OES analysis, successful doping of Gd3+ ions in CdS host has been proved. Fourier transform infrared spectroscopy results show the interaction of CTAB through headgroup at the nanoparticles surface. In the transmission electron microscopy images, it has been observed that the reaction temperature and Gd3+ doping played critical role on size and shape of nanocrystals. In UV–visible absorption as well as photoluminescence emission spectra, size and shape-dependent quantum confinement effect has been observed. On Gd3+ doping, surface states related emission peak shifted to higher wavelength, while intensity of peaks increased on increasing temperature. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Intensive research has been performed in the recent years on II–VI semiconducting nanocrystals (NCs) due to their tremendous size and shape-dependent optical responses [1,2]. The quantum confinement, which modifies density of states near band edge, realized an opportunity to tune the emission wavelength [3]. The large quantum efficiency in semiconductor nanocrystals rendered them successful candidate in light-emitting diodes [4], biological markers [5], and lasing materials [6]. While band edge recombination conventionally describes the luminescence in majority of semiconductors, the other luminescent peaks, due to presence of surface states play critical role on the optical properties of NCs. Due to high surface/volume ratio of quantum dots, coordination number of the atoms situated at surface is less compared to that of bulk, which creates defect states at the surface of the NPs. In quantum dots, the surface states significantly affect quantum efficiency and intensity of luminescent peaks. Among the varieties of semiconductors, CdS has the paramount importance due to its discrete energy levels and direct bandgap 2.42 eV at room temperature, thus showing numerous applications in LED, display devices, sensors, optoelectronic devices, photocatalysis, and others [7–10]. Doping semiconductor

∗ Corresponding author. Mobile: +91 8765583117. E-mail address: [email protected] (G. Pandey). http://dx.doi.org/10.1016/j.mseb.2015.06.007 0921-5107/© 2015 Elsevier B.V. All rights reserved.

nanocrystals with impurity metal ions has opened-up novel possibility of new kind of nanophosphors whose optical behavior may be different and superior as compared to the host material. The excellent photoluminescence properties of rare-earth ions make them potential candidate for doping in semiconductors to tune their optical properties [11]. It is understood that incorporation of impurity metal ions dramatically modify the physicochemical features of host materials [11,12], particularly at nanoscale, owing to confinement effect, which allow the tuning of defect ions by modulating the behavior of coupling with host lattice [13]. The sharp emission spectra of rare-earth ions, having high color purity of the emitted light due to their intra 4f-4f transitions, make them suitable candidate in photoluminescence displays, multicolor imaging and optoelectronic devices [14]. Further, the 4f orbital of rare-earth ions (e.g. Gd3+ under study) are shielded with 5s and 5p orbitals, the intra ff electronic transitions are supposed to be unaffected by host matrix and therefore by confinement. However, transition from excitonic level of host to energy levels of rare-earth ions exhibited low-energy peaks in emission spectra, thus influencing the optical properties of semiconducting NCs [15]. To achieve aforesaid practical utility of rare-earth ions doped CdS nanophosphors, high quality NCs, having controlled size, size distribution and morphology are required, and therefore a wellcontrolled synthetic strategy is highly desirable, particularly in aqueous phase synthesis. In the recent years, a number of papers have been published on synthesis and applications of CdS and trivalent rare-earth ions doped CdS NPs [16–24]. However, the few

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recent reports on Gd:CdS, prepared in organic phase [25], and in aqueous phase [26], have shown poor control on size and shape of NCs. The surface states, which act temporary traps for charge carriers, quench the radiative recombination and reduce the quantum yield. However, high quantum yield photoluminescence can be achieved by reducing nonradiative recombination of charge carriers via surface passivation [27]. Usually, in aqueous phase synthesis of colloidal NPs, suitable capping agents or stabilizer are utilized to increase monodispersivity and stability of NPs [9]. Therefore, using suitable surfactant, not only high quality and stable NCs can be prepared but also optical properties can be regulated by surface engineering. Hereunder in the present investigation, we have proposed a low cost and green synthetic route for preparation of Gd3+ ions-doped CdS NPs in aqueous medium, in presence of cationic surfactant CTAB and effect of reaction temperature on size, morphology, and optical properties of NCs has been discussed. 2. Experimental 2.1. Materials All the chemicals used in the present study, like cadmium acetate dihydrate; (Cd(CH3 COO)2 •2H2 O) gadolinium nitrate hexahydrate (GdNO3 •6H2 O), sodium hydroxide (NaOH), and cetyltrimethylammonium bromide (CTAB) were analytical grade reagents, purchased from Fisher Scientific. All the chemicals used in this investigation were used as received without further purification. Double distilled water and ethanol (purified by distillation) were used as solvents. 2.2. Synthesis In the present study, CdS and Gd:CdS NPs (Cd1−x Gdx S; x = 0 and 0.04) were synthesized in aqueous phase at 90 and 120 ◦ C in presence of CTAB. In a typical process, stoichiometric amount (x = 0 and 0.04) of total 5 mmol of reaction mixtures of (Cd(CH3 COO)2 •2H2 O and GdNO3 •6H2 O) were dissolved in 50 ml double distilled water. Aqueous solutions of CTAB (2 mmol in 20 ml double distilled water) were added to above two reaction mixtures. Now aqueous solution of thiourea; TU (20 mmol; 1.52 g in 100 ml of double distilled water) were prepared and made alkaline (pH 9) by adding 10−3 M NaOH. The alkaline solutions of TU were added to solution mixtures of metal ions. During course of reaction, the color of resultant solution turned yellowish. The above reaction mixtures were refluxed for half hour at 90 ◦ C in continuous supply of water cooling. After completion of reaction, the solutions were cooled naturally at room temperature, separated by ultracentrifugation and purified by repeat wash with ethanol. In order to examine the effect of reaction temperature on size and morphology of the products, the same experiments were performed at 120 ◦ C also, in otherwise the same reaction conditions.

amount of this solution was dropped on a copper grid and dried under a mercury lamp. ICP-OES analysis was performed on ICP-OES (ICAP 6300 Thermo Inc.) spectrometer using emission wavelength 257.610 nm. For ICP-OES analysis, samples were prepared using advanced microwave digestion system (ETHOS 1 MILESTONE, Italy) with a HPR-1000/10S high pressure segmented rotor. XRD patterns were recorded using PANalytical’s X’Pert Pro X-ray diffractometer in 2␪ range 20–80, using Cu K␣ radiation operated at 40 kV ˚ voltage and 30 mA current in step sizes of 0.02 ◦ C (d = 1.541 A). Surfactant adsorption on the NPs was studied by recording FTIR spectra on Perkin–Elmer Spectrum RXI in the wavelength region 4000–400 cm−1 . For FTIR analysis, 0.002 g of each sample was mixed with 0.2 g of KBr and then pelleted. PL measurements were carried out at 350 nm excitation wavelength on fluorescence spectrophotometer (Shimadzu RF-5300) with quartz cuvette of 1 cm path length. The optical properties of materials were studied by recording UV–vis absorption spectra on UV-2450 (Shimadzu) spectrophotometer having quartz cuvette of 1 cm path length. For both PL and UV–vis absorption spectral analysis, the NPs were dispersed in DMSO and thermostated at 25 ◦ C. The photoluminescence quantum yield PL QY () of CdS and Gd:CdS samples was measured at room temperature at the same excitation wavelength (350 nm using Xe lamp in RF-5300 in fluorescence spectrophotometer). An optically matched solution of dye Rhodamine B in ethanol (s = 0.96) was used as primary standard. In order to minimize readsorption between NPs, diluted solutions of NPs in DMSO were utilized. The PL QY was calculated using following equation [28]: ˚ = 0.96 ×

Isample Idye

×

Adye Isample

×

2DMSO 2ethanol

,

where Isample and Idye are the integrated areas of PL spectrum of sample and Rhodamine B, respectively, Asample and Adye are the absorbance of sample and Rhodamine B, respectively at excitation wavelength 350 nm, and  is the refractive index of solvents used. 3. Results and discussion XRD patterns of as prepared CdS and 4% Gd3+ ions-doped CdS NPs are shown in Fig. 1a. The XRD patterns were recorded to

2.3. Characterization Transmission electron microscopy (TEM) study was performed on H-7500 (Hitachi) transmission electron microscope. For TEM analysis, the samples were dispersed in ethanol and sonicated for 1 h to isolate the particles. A drop of the sonicated sample solution was added onto a carbon-coated copper grid, and the grid with the nanoparticles was dried in air. TEM micrographs were used to analyze particle size distribution. Particle size distribution analyses were carried out on freely available NIH ImageJ program. Energy dispersive X-ray spectra were recorded on Quanta 400 (from FEI Company) equipment. For EDX analysis, samples were dispersed in ethanol, using ultrasonic bath for 5 min. A small

Fig. 1. XRD patterns of as prepared CdS and Gd:CdS NPs in presence of CTAB at 90 ◦ C for half hour reaction time (a), shifting in XRD peak (b).

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Table 1 Reaction parameters, lattice parameters, particles size, bandgap, PL QY, and ICP-OES results of CdS and Gd:CdS NPs formed in the different reactions. Sample

Lattice parameter (Å)

Reaction temperature (◦ C)/time (hour)

Absorption peaks position (nm)/Eg (eV)

PL QY ()

Particle size calculated by Scherrer’s formula/TEM (nm)

Mole % determined by ICP-OES Gd

Cd

S

1. 3. 2.

CdS Gd:CdS CdS

90/1/2 90/1/2 120/1/2

457/2.96 429/3.22 438/3.14

0.320 0.761 0.416

7.3/8 ± 1.5 3.04/3–4 19.3/20 ± 2

– 0.66 –

76.73 76.13 76.77

23.25 23.19 23.21

4.

Gd:CdS

a = 5.78 a = 5.73 a = 3.89 c = 6.75 a = 3.85 c = 6.67

120/1/2

472/2.88

0.822

16.8/15–30

0.65

76.14

23.20

determine the phase, crystallinity and purity of as prepared CdS and Gd:CdS NPs. All the diffraction peaks in the XRD patterns of CdS NPs, synthesized at 90 ◦ C (Fig. 1b) show good agreement with standard cubic zinc blend CdS, due to presence of (110), (220), and (311) peaks (JCPDS file no. 10-454). In the XRD patterns of both samples, a low intensity peak, corresponding to (101) line of wurtzite CdS has also been detected, indicating presence of small amount of wurtzite CdS in the above samples. No other peak, corresponding to other materials has been detected, indicating the above samples crystallized predominantly in cubic CdS phase. The XRD peaks of Gd:CdS sample shifted toward higher 2␪ value compared to those of pure CdS. The upward shift in peak position is clearly visible in Fig. 1b, placed in inset. Since the size of Cd2+ ions (0.96 nm) is slight larger than that of Gd3+ ions (0.938 nm), the occupancy of smaller Gd3+ ions at Cd2+ lattice position is responsible for up-ward shift in peak position. It is known that hexagonal (wurtzite) phase is more stable than the cubic (zincblende) phase and the cubic phase changes to hexagonal phase at higher temperature [29]. All the diffraction peaks in the XRD patterns of CdS NPs, synthesized at 120 ◦ C (Fig. 2a) show good agreement with standard hexagonal wurtzite CdS, due to presence of (100), (002), (101), (102), (110), and (103) (JCPDS file no. 41–1049), indicating above sample is phase pure wurtzite CdS. Similar trend in peak shift, toward higher 2␪ value (Fig. 2b, placed in inset), has been observed in wurtzite Gd:CdS also, indicating successful doping of Gd3+ ions in CdS host. The lattice parameter for CdS and Gd:CdS at two reaction temperatures, presented in Table 1, indicate that on Gd3+ doping, lattice constants values have been decreased. Further, it has been observed that, in general, the diffraction lines of CdS and Gd:CdS samples, formed at 90 and 120 ◦ C are broad, indicating the formation of small sizes nanocrystals. However, the XRD peaks of CdS and Gd:CdS formed at 90 ◦ C are wider than those of at 120 ◦ C, indicating sizes of nanocrystals increased with increasing reaction temperature. Further, the intensity of peaks decreases on doping. The intensity decrease is more vigorous in case of Gd:CdS formed at 90 ◦ C than that of at 120 ◦ C.

Fig. 2. XRD patterns of as prepared CdS and Gd:CdS NPs in presence of CTAB at 120 ◦ C for ½ hour reaction time (a), shifting in XRD peak (b).

It is expected that this drastic increase in broadening of diffraction pattern and decrease in peak intensity in Gd:CdS nanocrystals occur not only due to decreased particles size but it arrives also, due to some kind of defects/distortion (like vacancies created, side disorder, dislocations etc.) created in the CdS lattice on doping. The phase compositions of Gd:CdS NCs, synthesized at 90 and 120 ◦ C, were determined using EDX spectra, shown in Fig. 3. From the EDX data the actual Gd contents were found to be 3.8 and 3.9%, respectively, which are close to the added amount (4%). From the EDX data successful doping of Gd3+ ions in CdS host has been proved. The elemental composition of synthesized CdS and Gd:CdS has been

Fig. 3. EDX spectra of Gd:CdS samples prepared at 90 and 120 ◦ C.

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Fig. 4. FTIR spectra of (a) pure CTAB; (b) CdS in presence of CTAB at 90 ◦ C; (c) Gd:CdS in presence of CTAB at 90 ◦ C; (d) CdS in presence of CTAB at 120 ◦ C, and (e) Gd:CdS in presence of CTAB at 120 ◦ C.

also analyzed by ICP-OES analysis (Table 1). The mole percentage of elements determined by ICP-OES analysis further confirms stochiometric composition of the products. The average size, of thus grown CdS crystallites, were calculated using Scherrer’s formula: D = 0.9/ˇcos,

(1)

where D is average calculated crystallite size of Cd1−x Gdx S NPs,  is the wavelength of Cu K␣ line (1.541 A◦ ), ˇ is the full width at half maxima (FWHM) of the peaks and  is the angle between incident beam and reflection lattice. The calculated particles sizes are presented in Table 1, which are in good agreement with particles size determined by TEM observations.

With a view to observe surfactant adsorption at the NPs surface, FTIR analysis of pure CTAB and Cd1−x Gdx S NPs (formed in presence of CTAB) were performed. The FTIR spectra are shown in Fig. 4 and all the observed results are summarized in Table 2. The presence of a small broad peak at 3410 cm−1 in FTIR spectrum of pure CTAB is correspond to O H stretching, due to some absorbed moisture [30]. The broadness and intensity of these peaks increased in presence of Cd1−x Gdx S NPs. This might be due to the presence of moisture of aqueous phase synthesized of Cd1−x Gdx S NPs. The spectrum of pure CTAB shows doublet peaks at 2849 and 2918 cm−1 , corresponding to symmetric and asymmetric -CH2 vibrations of alkyl chain [31]. The position of symmetric and asymmetric -CH2 vibrations remained almost unchanged in presence of all the Cd1−x Gdx S NPs samples, indicating no inter molecular interaction in the hydrophobic tail region. But the width of the band becomes narrower, suggesting hydrophobic (alkyl chain) tails have more ordered structures after getting bound with CdS NPs. The peaks corresponding to asymmetric and symmetric -CH scissoring vibrations of –N CH3 moiety of free CTAB at 1434 and 1462 cm−1 have been shifted and suppressed in between (sym 1540–1550 cm−1 and asym 1623–1625 cm−1 ) in presence of NPs, indicating the interaction of surfactant through head group. The singlet peak at 962 cm−1 in pure CTAB due to C H+ stretching, shifted in between 1025 to1008 cm−1 in presence of NPs. The considerable shift in frequency position is apparent due to interaction between N-containing group and Cd1−x Gdx S NPs surface [32]. This further indicates adsorption of surfactant molecules through headgroup. The -CH2 rocking doublet mode at 729 and 719 cm−1 in pure CTAB, appeared into singlet, in between 718 to 710 cm−1 , in presence of Cd1−x Gdx S NPs, indicating capping/more ordered arrangement of CTAB molecules after adsorption on NPs surface. The bands in between 477 and 584 cm−1 in the CdS samples are corresponding to Cd S lattice stretching vibrations [33], while in Gd–doped samples, the peaks corresponding to Cd-S stretching observed in between 477 to 620 cm−1 . Similar results, for CdS stretching was also reported by earlier by Thambidurai et al. [34]. The additional observed bands in the range of 437–530 cm−1 are probably due to induced distortion into CdS host lattice after doping. The bands observed in between 1115 to 1160 cm−1 might be due to Cd S Gd linkages. Since any separate peak has not been observed showing the presence of Gd3+ ions, it is expected that Cd2+ ions have been substituted by Gd3+ ions into CdS host lattice. This result is also supported by XRD, EDX and ICP-OES results. Fig. 5 shows TEM images of CdS NPs prepared in the aqueous phase at 90 ◦ C for ½ hour of reaction time in presence of CTAB. In the TEM images, isolated particles having spherical/ellipsoidal structures are observed. The average particle size of CdS NPs has been determined to be 8 ± 1.5 nm, which is in agreement with the particle size calculated using Scherrer’s formula (7.3 nm). Further careful observation of isolated particles in the TEM images indicates formation of core–shell like structures, probably due to formation of bilayer structure of cationic surfactant CTAB over CdS NPs surface [9]. This assumption is further supported by FTIR results also. Significant size reduction has been observed after addition

Table 2 Peak positions of various modes of vibration in FTIR spectra of pure CTAB, CdS, and Gd:CdS NPs formed in presence of CTAB in the different reactions. Peak position (cm−1 ) S.N.

Modes of vibration

Pure CTAB

CdS at 90 ◦ C

Gd:CdS at 90 ◦ C

CdS at 120 ◦ C

Gd:CdS at 120 ◦ C

1. 2. 3. 4. 5. 6.

O-H stretching due to moisture Sym&asym−CH2 vibrations of aliphatic chain Asy&Sym CH+ scissoring vibration of CH3 -N moiety C-N+ stretching band -CH2 rocking doublet Cd-S stretching/stretching due to induced defects states

3410 2849–2918 1484–1462 962 729–719 –

3414 2854–2921 1623–1540 1025 710 600–500

3383–3198 2855–2921 1625–1548 1008 718 477

3414 2854–2956 1623–1540 1025 715 600

3385–3189 2846–2920 1622–1549 1011 717 600–500

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Fig. 5. TEM images of CdS NPs formed at 90 ◦ C.

Fig. 6. TEM images of Gd:CdS NPs formed at 90 ◦ C.

of 4% Gd3+ ions in the CdS host in otherwise the same reaction conditions. TEM images show the formation of be 3–4 nm size Gd:CdS NPs 90 ◦ C (Fig. 6). Further, it has been observed that the so formed NPs are loosely interlinked due to high surface energy and appeared as clouds, wherein Gd:CdS NPs are impregnated. This observation suggests that each cloud is composed of monodispersed Gd:CdS NPs. Dramatic shape and size variation has been observed on increasing reaction temperature. In the TEM images (Fig. 7), 20 ± 2 nm diameter triangular–shape CdS NPs are formed at 120 ◦ C, in otherwise same reaction conditions. At some places, the triangular–shaped CdS structures stacked in staggered manner, forming star–like CdS structures. Further careful observation indicates that the core of triangular/star shaped CdS NPs are rear, while the central portion is dense. The enlarged view of TEM image (Fig. 7c), indicates the formation of bilayer structure. In the FTIR results, it has been found that the surfactant CTAB adsorbed at

the NPs surface through its headgroup. Since the hydrophobic tail cannot prefer aqueous phase, the CTA+ ions present in the bulk solution attached with adsorbed CTA+ through tail. Tail-to-tail attachment results orientation of hydrophilic headgroup toward aqueous phase, thus forming bilayer structure of surfactant over NPs surface. On addition of 4% Gd3+ ions, almost monodispersed, ∼25 nm diameter spherical Gd:CdS NPs are formed at 120 ◦ C reaction (Fig. 8). It is, therefore, obvious that reaction temperature and Gd3+ ions doping in CdS host play critical role on size and morphology of CdS NCs. Particle size distribution histograms of CdS and Gd:CdS NPs, formed at 90 and 120 ◦ C reactions have been presented in Fig. 9. As obvious from the histograms, on increasing reaction temperature, NPs with comparatively fair size distribution have been formed. Its known that S−2 ions, produced by alkaline hydrolysis of thiocarbamide, react with Cd2+ ions to form CdS [35]. In suitable reaction conditions, CdS NPs are formed where capping groups

Fig. 7. TEM images of CdS NPs formed at 120 ◦ C.

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Fig. 8. TEM images of Gd:CdS NPs formed at 120 ◦ C.

or surfactants play an important role in growth of NPs. It is supposed that the higher temperature reaction (120 ◦ C) is thermodynamically controlled and favors wurtzite-phase seeds, while at the lower temperature (90 ◦ C), the reaction is kinetically driven, engendering a zinc blende-phase ‘core’ [29]. Further, at higher temperature 120 ◦ C, the initial nucleation rate is high and subsequent growth rate is low. At lower temperature 90 ◦ C, 8 ± 1.5 nm diameter spherical/ellipsoidal CdS NPs are formed since the reaction is kinetically driven, depending upon reactants concentration and controlled of surfactant CTAB, whereas at higher temperature 120 ◦ C, the reaction is thermodynamically controlled (Scheme 1). Formation of triangular CdS at higher temperature 120 ◦ C can be explained by Ostwald ripening (OR) process, formally described by Lifshiz–Slyozov–Wagner (LSW) theory [9], according to which initially formed, smaller and more soluble NCs dissociate and grow in form of larger NCs. It is expected that at higher temperature Gd3+ ions, triangular CdS are formed in the Ostwald ripening process. When 4% Gd3+ ions are added in the reaction mixture, it is supposed that Gd3+ ions compete with Cd2+ ions, to be crystallized into CdS host. Therefore, in presence of Gd3+ -ions reaction became sufficiently slow to form 3–4 nm diameter CdS NPs. Though the stabilizer CTAB present in the reaction medium effectively terminates or ‘caps’ the CdS NPs formed at 90 ◦ C having high surface energy however, the same amount of surfactant CTAB (2 mmol) has to cap or stabilize the larger number of smaller Gd:CdS NPs formed at

Fig. 9. Particle size distribution histogram of (a) CdS NPs formed at 90 ◦ C; (b) Gd:CdS NPs formed at 90 ◦ C; (c) CdS NPs formed at 120 ◦ C, and (d) Gd:CdS NPs formed at 120 ◦ C.

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Scheme 1. Schematic representation of evolution of CdS and Gd:CdS NPs.

same reaction temperature. Therefore, an imbalance surface energy remained left, making NPs ‘unstable’ and still to be stabilized. In this situation, in order to stabilize these ‘unstable’ Gd:CdS NPs, loosely interlinked assemblies are formed at 90 ◦ C, as shown in Fig. 8. At higher temperature 120 ◦ C, larger size (∼ 25 nm diameter) Gd:CdS NPs are formed in the process of Ostwald ripening. The UV–visible spectra of all the samples (Fig. 10), synthesized at 90 and 120 ◦ C, respectively has been recorded by dispersing NPs in absolute alcohol, and all the recorded observations have been summarized in Table 1. The CdS NPs synthesized at 90 and 120 ◦ C exhibit absorption shoulder at 457 and 438 nm, respectively. The blue shift in absorption peak, compared to bulk CdS (at 512 nm), indicates quantum confinement effect due to their small size. Quantum confinement effect is expected when size of NPs becomes comparable to or smaller than Bohr radius of excitons, giving rise to blue shift in absorption peak related to bandgap. In case of Cds such quantum confinement observed when size of crystallites reduces to 5 nm which is approximate exiton diameter of CdS [36]. Generally it is believed that quantum confinement takes place when particle size becomes comparable to bulk exiton Bohr radius, however, in CdS the blue shift in bandgap versus size has also been observed beyond the quantum confinement regime [37]. It is further noticed that in case of bigger triangular CdS NPs (20 ± 2 nm size, formed at 120 ◦ C), the blue shift was larger (at 438 nm) compared to that of spherical/ellipsoidal smaller CdS NPs (8 ± 1.5 nm diameter), where absorption shoulder was at 457 nm. This is because of triangular shape of CdS NPs. Since the optical properties of non-spherical NCs are determined by their smallest available dimension. The edge tips of triangular CdS NPs are sufficiently thin to observe large quantum confinement effect [38]. In case of Gd:CdS NPs, synthesized at 90 and 120 ◦ C, the absorption shoulder observed at 429 and 472 nm, respectively, which are also in agreement with their particle sizes.

Fig. 10. UV–visible absorption spectra of CdS and Gd:CdS NPs formed in presence of CTAB at 90 and 120 ◦ C for half hour reaction time.

Fig. 11. PL spectra of CdS and Gd:CdS NPs formed in presence of CTAB at 90 and 120 ◦ C for ½ hour reaction time at 350 nm excitation wavelength.

Since CdS is a direct bandgap semiconductor, the bandgap values of all samples have been calculated using the relation: ˛h = A(h−Eg )1/2 ; where ˛ is the absorption coefficient, h the photon energy, A is a constant and Eg is the band gap value. (˛h)2 versus h plot (Tauc plots) has been used to calculate Eg values of different Cd1−x Gdx S samples. The intercepts of extrapolated straight lines of the linear portion of the curves give the Eg values of various samples (Table 1). Cds NPs, prepared in colloids, generally exhibit two different kind of emission bands. First, the band edge emission, which is strongly dependent on particle size and second the lowenergy emission (∼500–700 nm), due to recombination of excitons, trapped in surface states [39,40]. The photoluminescence spectra of pure and Gd3+ ions doped CdS NPs synthesized at 90 and 120 ◦ C were recorded at 350 nm excitation wavelength (Fig. 11). In case of pure CdS, synthesized at 90 ◦ C, the band centered at 512 nm is attributed to recombination of trapped electrons/holes or excitons in surface states, which is resulted due to small size of nanocrystals. When reaction temperature was increased at 120 ◦ C, the size of NCs increased whereas the position of emission peak shifted to lower wavelength side (at 493 nm), due to shape-dependent quantum confinement effect of thin edge tip triangular CdS NCs. Simultaneously intensity of band increased because of increased availability of ‘trapes’ at higher reaction temperature. A large red shift and intensity enhancement have been observed when 4% Gd3+ ions were incorporated into CdS host. In case of Gd:CdS formed at 90 ◦ C, the emission band shifted toward higher wavelength side at 539 nm, possibly due to transition taken place between excitonic level of CdS and energy levels of Gd3+ [16]. On increasing reaction temperature at 120 ◦ C, the peak position of Gd:CdS was unaltered, however, the intensity of peak was almost doubled possibly due availability of larger amount of Gd3+ related surface states. The PL QY of CdS and Gd:CdS NPs are presented in Table 1. At 90 ◦ C, the PL QY of CdS NPs was 0.320, while at elevated temperature 120 ◦ C it was increased to 0.416, probably due to decrease in non-radiative traps and defect centers of larger sized triangular CdS. On incorporation of 4% Gd3+ ions, a large increase in PL QY has been observed. In case Gd:CdS at 90 ◦ C, the PL QY was found to be 0.761, while for Gd:CdS at 120 ◦ C, it was 0.822. Such large increase in PL QY was attributed due to significant diminishing of non-radiative recombination and enhancement of radiative recombination, caused by doping.

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4. Conclusion In summary, CdS and Gd:CdS NPs have successfully been synthesized adopting a simple, low cost and environmentally green synthetic route in aqueous medium. At low temperature 90 ◦ C, 8 ± 1.5 nm diameter spherical/ellipsoidal CdS are formed because reaction is kinetically driven, dependent on reactants concentration and surfactant, while at higher temperature 120 ◦ C, larger size (∼ 20 nm) triangular CdS produces in process of OR. On addition of Gd3+ ions, rate of reaction slowed down, producing smaller size (3–4 nm sizes) Gd:CdS at 90 ◦ C, however, at higher temperature 120 ◦ C larger size (∼25 nm diameter) spherical Gd:CdS formed due to OR. Therefore, it is concluded that reaction temperature and Gd3+ doping played an important role on shape and size of CdS and Gd:CdS NCs. Shifts in XRD peaks at higher 2␪ value in Gd:CdS at both reaction temperatures indicate successful doping of Gd3+ ions in CdS host lattice. Presence of Gd in stoichiometric ratio has been confirmed by EDX spectra and ICP-OES results. FTIR spectra of pure and Cd1−x Gdx S NPs adsorbed CTAB indicate that the peak position of surfactant tail region remained unaffected however, peak positions of headgroup, like -CH scissoring vibrations of N CH3 moiety, C H+ stretching, -CH2 rocking etc., have significantly been changed, indicating adsorption of surfactant CTAB through its headgroup at the NPs surface. Moreover, additional peaks of CTAB capped NPs in 500–638 and 1115–1160 cm−1 region indicate substitution of Cd2+ by Gd3+ in CdS host lattice. In the UV–visible absorption spectra, large blue shifts in absorption shoulder of CdS and Gd:CdS, compared to bulk, indicated the quantum confinement effect, caused by size and shape of NCs. In the PL spectra, surface states related emission peak of CdS shifted at lower wavelength even after increasing particle size at 120 ◦ C, due to shape-dependent quantum confinement effect. On Gd3+ doping at 90 ◦ C, a large red shift in emission peak was observed, due to transition between excitonic level of CdS and energy levels of Gd3+ , however, at 120 ◦ C, the peak position of Gd:CdS unaltered but the intensity of peak almost doubled due to availability of larger amount of Gd3+ -related surface states at higher temperature. On incorporation of 4% Gd3+ ions, a large increase in PL QY has been observed, due to diminishing of non-radiative recombination and enhancement of radiative recombination, caused by doping. References [1] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59–61. [2] J. Hu, L.-S. Li, W. Yang, L. Manna, L.-W. Wang, A.P. Alivisatos, Science 292 (2001) 2060–2063.

[3] [4] [5] [6] [7] [8]

[9] [10] [11] [12]

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

A.E. Ekimov, A.A. Onushchenko, JETP Lett. 34 (1981) 345–349. Y. Taniyasu, M. Kasu, T. Makimoto, Nature 441 (2006) 325–328. A. Kishimoto, Y. Inou, T. Kita, O. Wada, Phys. Status Solidi C 4 (2007) 2490–2493. M. Maqbool, I. Ahmad, H.H. Richardson, M.E. Kordesch, Appl. Phys. Lett. 91 (2007) 193511–193513. S. Mandal, D. Rautaray, A. Sanyal, M. Sastry, J. Phys. Chem. B 108 (2004) 7126–7131. L. Weinhardt, T. Gleim, O. Fuchs, C. Heske, E. Umbach, M. Bar, H.-J. Muffler, C.-H. Fischer, M.C. Lux-Steiner, Y. Zubavichus, T.P. Niesen, F. Karg, Appl. Phys. Lett. 82 (2003) 571–573. G. Pandey, S. Dixit, J. Phys. Chem. C 115 (2011) 17633–17642. N. Zhang, M.-Q. Yang, Z.-R. Tang, Y.-J. Xu, ACS Nano 8 (2014) 623–633, 2014. B.C. Cheng, X.M. Yu, H.J. Liu, M. Fang, J.D. Zhang, J. Appl. Phys. 105 (2009) 14311–14315. J.H. Yu, X.Y. Liu, K.E.J. Kweon, J.J. Park, K.T. Ko, D.W. Lee, S.P. Shen, K. Tivakornsasithorn, J.S. Son, J.H. Park, Y.W. Kim, G.S. Hwang, M. Dobrowolska, J.K. Furdyna, T. Hyeon, Nature Mater. 9 (2010) 47–49. M.P. Pileni, Catal. Today 58 (2000) 151–166. H. Jianhua, H. Guogen, H. Xiongwu, W. Rui, J. Rare Earths 24 (2006) 728–731. L. Saravanan, R. Jayavel, A. Pandurangan, J.-H. Liu, H.-Y. Miao, Mater. Res. Bull. 52 (2014) 128–133. X.D. Li, J. Zhang, Q. Xiong, ACS Nano 6 (2012) 5283–5290. L. Saravanan, L.R. Jayavel, A. Pandurangan, J.-H. Liu, H.-Y. Miao, Powder Technology 266 (2014) 407–411. K.D. Nisha, M. Navaneethan, Y. Hayakawa, S. Ponnusamy, C. Muthamizhchelvan, J. Alloys Compd. 509 (2011) 5816–5821. S. Das, K.C. Mandal, Mater. Lett. 66 (2012) 46–49. X. Wang, D. Li, Y. Guo, X. Wang, Y. Du, R. Sun, Opt. Mater. 34 (2012) 646–651. J. Lian, Y. Xu, M. Lin, Y. Chan, J. Am. Chem. Soc. 134 (2012) 8754–8757. M.-Q. Yang, N. Zhang, M. Pagliaro, Y.-J. Xu, Chem. Soc. Rev. 43 (2014) 8240–8254. A.C.A. Silva, S.W. da Silva, P.C. Morais, N.O. Dan, ACS Nano 8 (2014) 1913–1922. E.S.F. Neto, S.W. da Silva, P.C. Morais, N.O. Dantas, J. Phys. Chem. C 117 (2013) 657–662. K. Kaur, G.S. Lotey, N.K. Verma, J. Mater. Sci. Mater. Electron. 25 (2014) 311–316. M. Thambidurai, N. Muthukumarasamy, D. Velauthapillai, C. Lee, J. Mater. Sci. Mater. Electron. 24 (2013) 4535–4551. L. Saravanan, S. Diwakar, R. Mohankumar, A. Pandurangan, R. Jayavel, Nanomater. Nanotechnol. 1 (2011) 42–48. J.N. Demasa, G.A. Crosby, J. Phys. Chem. 75 (1971) 991–1024. W.Y. Jun, S.M. Lee, N.J. Kang, J.W. Cheon, J. Am. Chem. Soc. 123 (2001) 5150–5151. R.P. Sperline, Langmuir 13 (1997) 3715–3726. R.P. Sperline, Y. Song, H. Freiser, Langmuir 13 (1997) 3727–3732. N.V. Venkataraman, S. Vasudevan, J. Phys. Chem. B 105 (2001) 1805–1812. Q. Li, V. Kumar, Y. Li, H. Zhang, T.J. Marks, R.P.H. Chang, Chem. Mater. 5 (2005) 1001–1006. M. Thambidurai, N. Murugan, N. Muthukumarasamy, S. Agilan, S. Vasantha, R. Balasundaraprabhu, J. Mater. Sci. Technol. 26 (2010) 193–199. G. Pandey, S. Shrivastav, H.K. Sharma, Physica E 56 (2014) 386–392. C. Unni, D. Philip, K.G. Gopchandran, Spectrochimica Acta Part A 71 (2008) 1402–1407. A. d’Andrea, R. Del Sole, Phys. Rev. B 41 (1990) 1413–1423. K.K. Nanda, S.N. Sahu, Adv. Mater. 13 (2001) 280. Z.H. Zhang, W.S. Chin, J.J. Vittal, J. Phys. Chem. B 108 (2004) 18569–18574, 108. W.W. Yu, X.G. Peng, Angew. Chem. Int. Ed. 41 (2002) 2368–2371.